The biology of reconstitution of T cell populations following acute loss remains incompletely characterized. Using murine models, we first identified two primary pathways of T cell immune reconstitution, the classic, thymic-dependent pathway, and a second, thymic-independent pathway. We then identified T cell surface markers which allowed identification, by phenotyping of reconstituted T cell populations, of the pathways which had given rise to them, and then applied this information to the characterization of T cell reconstitution in patients. This work showed an essential role for the thymus in regenerating CD4+ T cells quantitatively. CD8+ T cells can numerically be reconstituted by peripheral expansion for immune reconstitution, but both CD4+ and CD8+ T cells require thymic activity for maintenance or regeneration of repertoire diversity. These findings led in turn to a research emphasis on understanding mechanisms which control thymic function, and new treatments to treat cancer in the setting of a regenerating immune system. This work has progressed to the development of two new project areas of research -- one focused on points of regulation of thymus function and one on introducing agents into clinical trials. The work addressed in this project has also led to efforts in investigating IL-7 effects on the maturation of thymocytes. We have identified IL-7 as a negative regulator of thymopoiesis as well as being essential for thymocyte development. These dual roles are dose dependent and negative regulation at higher concentrations is mediated through control of Notch signaling -- which is central to T/B lineage commitment. At high doses, IL-7 favors B cell development and so turns the thymus towards a B cell-poietic organ. Additionally, we have worked to identify genes which might regulate the thymus and have characterized a gene called Tbata (previously SPATIAL) which is a negative regulator acting within the stromal cell compartment. It appears to exert its effect through control of cell cycle, specifically through regulation of the Nedd8 pathway. Recent results indicate that regulation of cell cycle by Tbata extends beyond Nedd8 and involves p53. This is due to involvement of the protein encoded by Tbata in a large multi-protein complex which controls gene expression. Interestingly, this may be reflected in differences of thymopiesis in female versus male mice. The difference is now clear from multiple experiments, and the mechanism is under investigation. The role of IL-7 as a possible regulator of thymus function was noted above; its role in peripheral homeostasis has been further investigated by characterizing IL-7 receptor regulation among T cell subsets. The work with IL-7 receptor modulation has shown a complex pattern of differential signaling regulation among subsets with the apparent effect of favoring the sustaining of primitive naive T cells. This is of special interest because it provides a basis for understanding how a single cytokine such as IL-7 can differentially regulate multiple distinct T cell subsets. Specifically, there is differential regulation between naive CD4 T cells and memory CD4 T cells. Differences in signaling distal to the receptor for IL-7 appear to mechanistically account for this differential regulation. We are now addressing the biology of signaling by the two primary homeostatic cytokines in the CD8 naive versus CD8 memory subsets in murine models and have found again differences at the level of T cell receptor turnover. We have also made the observation that in T cell immune reconstitution reliant on the thymus-independent pathway in humans, that such expansion results in premature aging of the T cells with resultant cellular senescence, a likely contributor to T cell functional impairment for years following transplant. Investigating the stem cell compartment in murine models, we showed that FLT3 ligand regulates thymic precursor cells and hematopoietic stem cells through interactions with CXCR4 and the marrow niche. Finally, a series of experiments using deuterated glucose to assess T cell subsets kinetics in the pathogenesis of chronic GVHD in murine models yielded information on such subsets and evolved into efforts to successfully image rapidly proliferating cells in that disease.